The present invention relates to a joint for connecting an open end of a tubular ceramic element to a tubesheet. More particularly, the present invention relates to such a joint in which an internal seal housing is connected to the tubular ceramic element and an external seal housing, connected to the tubesheet, retains the internal seal housing and therefore, the tubular ceramic element.
Tubular ceramic elements such as candle filters and oxygen transport membranes are supported by metal tubesheets within devices such as filter assemblies and reactors, respectively, that are designed to operate at high temperature. The joining of such ceramic elements to tubesheets in a manner that provides a seal at the juncture is very problematical due to the dissimilarity in the thermal expansion characteristics of ceramic and metallic materials and the high temperature environment in which such elements operate.
For instance, in a reactor that contains oxygen transport membrane tubes, the oxygen transport membrane tubes are supported by a tubesheet. An oxygen containing gas is exposed to a cathode side of the membrane where oxygen gains two electrons to become ionized. The ceramic material making up the membrane transports the oxygen ions to an opposite anode side where electrons are lost to the membrane and the oxygen ions reconstitute to form elemental oxygen which can be collected or reacted to produce reaction components. Depending upon the reactor design the oxygen containing gas is either introduced into the tubes or to the outer surfaces of the tubes with the elemental oxygen or reaction products being extracted from the opposite surfaces of the tubes. In order to effect such a process, the oxygen containing gas and the elemental oxygen or reaction components to be collected are segregated by the tubesheet and therefore, the juncture of the oxygen transport membrane tubes and the tubesheet must incorporate a seal.
An example of such a seal can be found in EP 106 732 0 A2 in which a gas type seal is formed by utilizing one or more compliant metallic, torus-like ring sealing elements. In the seal disclosed in this patent, a ceramic membrane tube is held and sealed within an external seal housing by way of the ring sealing elements that is force fit into a second passageway of the external seal housing
U.S. Pat. No. 5,401,406 discloses a seal for a filter element to connect the filter element to a tubesheet. The filter element has an enlarged end that fits within a second passageway of the tubesheet. A disc-like element bears against compressible, sealing material located at the open end of the filter element and between the filter element and the tubesheet. The disc-like element is attached to the tubesheet, by means such as by welding, to function as a hold down element to hold the filter element in place, sealed against the tubesheet and sealed against the hold down element.
In all of these foregoing references, the seal between the tubular ceramic element and the tubesheet, the ceramic-to-metal seal, is produced during assembly of the ceramic elements and the tubesheet. As mentioned above, it is difficult to make reliable cermic-to-metal seals in the first instance. This sealing problem becomes particularly troublesome when many tubular ceramic elements are to be attached to a tubesheet. For instance, during assembly, when long ceramic elements are maneuvered into proper position relative to the tubesheet, great care must be taken to not damage the ceramic elements while at the same time effecting a seal at each juncture of the ceramic elements and the tubesheet. Furthermore, such assembly only allows for the testing of the ceramic-to-metal seal after assembly. If there are defective seals, individual elements must be removed and reassembled.
As will be discussed, the present invention provides a joint between a tubular ceramic element and a tubesheet that incorporates a ceramic-to-metal seal that does not have the foregoing assembly problems because such seal is formed prior to attachment of the tubular ceramic element to the tubesheet itself.
The present invention provides a joint for connecting a tubular ceramic element to a tubesheet. The joint has a metallic, internal seal housing to retain the tubular ceramic element and a metallic, external seal housing connected to the tubesheet to retain the internal seal housing. The ceramic-to-metal seal is located within the internal seal housing to seal the tubular ceramic element within the internal seal housing. A less troublesome metal-to-metal seal is located within the external seal housing to seal the internal seal housing within the external seal housing.
The internal seal housing has a first passageway communicating between opposite open ends thereof. The first passageway is configured to receive an open end portion of the tubular ceramic element such that the tubular ceramic element projects from one of the opposite open ends and an interior region of the tubular ceramic element is in flow communication with the other of the opposite open ends. The internal seal housing has an internal sealing surface defined in the first passageway and is assembled in sections to allow the tubular ceramic element to be inserted into the internal seal housing with the open end portion of said tubular ceramic element located adjacent to the sealing surface.
The ceramic-to-metal seal is situated between the internal sealing surface and the open end portion of the tubular ceramic element to seal the first passageway and to hold the tubular ceramic element in place.
The external seal housing is connected to an opening in the tubesheet and is configured to hold the internal seal housing within a second passageway thereof. The second passageway is in flow communication with opposite sides of said tubesheet. At least one metal-to-metal seal is located within the second passageway and between the internal seal housing and an interior surface of the second passageway.
Since the internal seal housing is assembled in sections and contains the ceramic-to-metal seal, such seal can be perfected for each of the tubular ceramic element prior to their being joined to the tubesheet. After assembly and suitable testing of this seal, the internal seal housing is then inserted into the external seal housing when the metal-to-metal seal is effected between such components. Since, the metal-to-metal seal presents a less difficult sealing problem that the ceramic-to-metal seal, the joinery of the present invention allows for a more certain assembly of the tubular ceramic elements and the tubesheet with fewer sealing defects than the prior art.
Advantageously, the sections of the internal seal housing can be formed by first and second sections having first and second passageway portions, respectively, to form the first passageway. In such case, the internal sealing surface is defined in the first passageway portion. The open end portion of the tubular ceramic element can be provided with an outwardly flared, conical-like configuration and the internal sealing surface can also be of conical configuration, complimenting the outwardly flared, conical-like configuration of the open end portion. A sealing material is located between said open end portion of the tubular ceramic element and the internal sealing surface. Assembly of the first and second sections drives the open end portion against the sealing material and the internal sealing surface to effect the ceramic-to-metal seal and to hold the tubular ceramic element in place, retained within the internal seal housing.
Preferably, the ceramic-to-metal seal can further comprise an internal washer-like gasket located between the open end portion of the tubular ceramic element and the second section of the internal seal housing at a terminus of the second passageway portion thereof. Advantageously, the first and second sections of the internal seal housing are connected to one another by a threaded engagement.
The second passageway of the external seal housing can terminate, at one end thereof, in an annular shoulder defining an aperture through which the tubular ceramic element projects. The annular shoulder also defines the interior surface for sealing. An external washer-like gasket is located between an undersurface of the first section of the internal seal housing and the interior surface defined by the annular shoulder. A retaining element threadably engages a threaded end of the external seal housing, opposite to the aperture, to bear against the internal seal housing and drive the internal seal housing against the external washer-like gasket to seal the internal seal housing within the external seal housing, thereby to effect the at least one metal-to-metal seal.
The external washer-like gasket can be a first of two external washer-like gaskets. The second section of the internal seal housing can be provided with an annular flange positioned to overlie the threaded end of the external seal housing. The retaining element bears against the annular flange and a second of the two external washer-like gaskets, located between the annular flange and the opposite end of the internal seal housing, to provide a seal between the annular flange and the opposite end of the internal seal housing. An annular space is defined between the internal seal housing and the external seal housing. The external seal housing can have an inlet to introduce a buffer gas in the annular space and the first section of the internal seal housing can be ported to allow the buffer gas to pressurize the first passageway portion of said first passageway. In such manner, the buffer gas prevents leakage of process fluid of the particular process of concern through the ceramic-to-metal seal and therefore, the tubesheet.
When a buffer gas is used, in order to ensure proper sealing of the external seal housing, the internal seal housing can further be provided with a third, spacer section configured to threadably engage the first section of the internal seal housing and to contact the annular surface of the external seal housing. This threaded engagement allows the overall length of the internal seal housing to be adjusted to aid in ensuring proper sealing at the external seals provided by first and second external washer-like gaskets.
In any embodiment of the present invention, the sealing material for the ceramic-to-metal seal can be a rope seal.
Specific embodiments of the present invention are provided to ensure perfection of the seal located at the internal gasket-like washer. In one embodiment, the open end portion of the tubular ceramic element can be provided with a spherical end section. The second passageway portion of the first passageway in such embodiment has an annular recessed surface at the terminus of the second passageway portion of the first passageway. The annular recessed surface has a stepped, chamfered configuration to center the internal gasket-like washer with respect to the second passageway portion of the first passageway and such that the internal gasket-like washer bears against the spherical end section of the tubular ceramic element to align said tubular ceramic element with the second passageway portion of the first passageway.
In another embodiment of the present invention to ensure proper sealing at the internal gasket-like washer, the annular surface at the terminus of the second passageway portion of first passageway is a conical recessed surface. A metallic washer is interposed between the internal washer-like gasket and the annular surface. The metallic washer has an outer, curved transverse cross-section to center the metallic washer with respect to the second passageway portion of the first passageway.
Any of the internal or first and second external washer-like gaskets can be formed of graphite, asbestos, vermiculite, or boron nitrite contained in a metal jacket.